U.S. patent number 7,459,202 [Application Number 11/428,454] was granted by the patent office on 2008-12-02 for printed circuit board.
This patent grant is currently assigned to Motorola, Inc.. Invention is credited to Gregory J. Dunn, Jaroslaw A. Magera.
United States Patent |
7,459,202 |
Magera , et al. |
December 2, 2008 |
Printed circuit board
Abstract
A sequentially laminated printed circuit board having highly
reliable vias can be fabricated by pattern plating flanges or via
lands on a copper foil, laminating the foil to a prepreg so that
the flanges are embedded into the surface of the prepreg, creating
via holes in the laminate that are substantially concentric with
the individual flanges, plating the via holes with copper,
chemically or mechanically milling off a portion of the copper
plating and optionally some of the copper foil to reduce the
overall thickness of the laminate, and laminating a second and
optionally a third prepreg to the laminate. The resulting printed
circuit board has the flanges embedded in the surface of the
laminate so that the inside wall of the flange is electrically and
mechanically attached to the outside wall of the plated through
hole barrel.
Inventors: |
Magera; Jaroslaw A. (Palatine,
IL), Dunn; Gregory J. (Arlington Heights, IL) |
Assignee: |
Motorola, Inc. (Schaumburg,
IL)
|
Family
ID: |
38877016 |
Appl.
No.: |
11/428,454 |
Filed: |
July 3, 2006 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20080003414 A1 |
Jan 3, 2008 |
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Current U.S.
Class: |
428/209; 174/250;
174/255; 174/261; 174/264 |
Current CPC
Class: |
H05K
1/116 (20130101); H05K 3/428 (20130101); H05K
3/0008 (20130101); H05K 3/0047 (20130101); H05K
3/025 (20130101); H05K 3/205 (20130101); H05K
3/427 (20130101); H05K 2201/09063 (20130101); H05K
2201/09918 (20130101); H05K 2203/1438 (20130101); Y10T
428/24917 (20150115) |
Current International
Class: |
B32B
15/00 (20060101); H05K 1/00 (20060101) |
Field of
Search: |
;174/260-266,250-255
;361/792-795 ;428/209 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lam; Cathy
Attorney, Agent or Firm: Lamb; James Davis; Valerie M.
Claims
What is claimed is:
1. A printed circuit board, comprising: a deformed thermoset
dielectric medium, comprising a polymer, having a major surface;
one or more apertures formed in the deformed thermoset dielectric
medium, each aperture having a layer of copper deposited on
aperture walls thereof; a copper flange embedded into the major
surface of the deformed thermoset dielectric medium and
substantially concentric to a central axis of the aperture, wherein
interior vertical walls of the flange are attached to an outer
portion of the layer of copper on the aperture wall,wherein the
thickness of the copper flange is equal to or greater than two
times the thickness of the layer of copper on the aperture walls;
and a copper foil situated on at least portions of the major
surface of the deformed thermoset dielectric medium, with portions
of the copper foil overlying the copper flange.
2. The printed circuit board as described in claim 1, wherein the
deformed thermoset dielectric medium further comprises glass fiber
reinforcement in the polymer.
3. The printed circuit board as described in claim 1, further
comprising an additional layer of dielectric medium on a surface of
the deformed thermoset dielectric medium opposite the major
surface.
4. The printed circuit board as described in claim 3, wherein the
aperture is filled with portions of the additional layer of
dielectric medium.
5. The printed circuit board as described in claim 4, further
comprising an additional copper plating situated on the filled
portion of the aperture, and on the copper foil.
6. The printed circuit board as described in claim 1, wherein the
copper foil situated on at least portions of the major surface of
the deformed thermoset dielectric medium comprises circuit
conductors.
7. The printed circuit board as described in claim 1, wherein the
copper flanges are shaped as diamond, triangular, square,
rectangular, pentagonal, hexagonal, octagonal, round, elliptical,
or polygonal.
8. The printed circuit board as described in claim 1, wherein the
thickness of the copper flange is greater than 1.0 mil and greater
than the thickness of the layer of copper on the aperture
walls.
9. A printed circuit board, comprising: a glass-reinforced
thermoset polymer substrate, having a major surface; one or more
apertures formed in the glass-reinforced thermoset polymer
substrate, each aperture having a layer of copper deposited on
aperture walls thereof, and filled with the polymer; a copper
flange embedded into the major surface of the glass-reinforced
thermoset polymer substrate and substantially concentric to a
central axis of the aperture, wherein interior vertical walls of
the flange are attached to an outer portion of the layer of copper
on the aperture wall, and wherein the thickness of the copper
flange is greater than 1.0 mil and equal to of greater than 1.5
times the thickness of the layer of copper on the aperture wall;
and a copper foil situated on at least portions of the major
surface of the glass-reinforced thermoset polymer substrate, with
portions of the copper foil overlying the copper flange.
10. The printed circuit board as described in claim 9, wherein the
copper foil situated on at least portions of the major surface of
the glass-reinforced thermoset polymer substrate comprises circuit
conductors.
11. The printed circuit board as described in claim 9, further
comprising an additional layer of a glass-reinforced thermoset
polymer substrate on a surface of the glass-reinforced thermoset
polymer substrate opposite the major surface.
12. The printed circuit board as described in claim 9, wherein the
shape of the copper flanges is selected from the group consisting
of diamond, triangular, square, rectangular, pentagonal, hexagonal,
octagonal, round, elliptical, and polygonal.
13. The printed circuit board as described in claim 9, further
comprising an additional copper plating situated on the filled
portion of the aperture, and on the copper foil.
14. A printed circuit board, comprising: a first glass-reinforced
thermoset polymer substrate having a plurality of
plated-through-holes therein, at least some of the
plated-through-holes having a copper collar situated around an
upper periphery thereof and embedded in the surface of the
substrate such that an interior wall of the collar is attached to
an exterior wall of a barrel of the plated-through-hole, and such
that the thickness of the copper collar is greater than 1.0 mil and
equal to or greater than 1.5 times the the thickness of the barrel;
a copper foil attached to a non-embedded portion of the collar and
to portions of the surface of the first glass-reinforced thermoset
polymer substrate; and a second glass-reinforced thermoset polymer
substrate laminated to an opposing surface of the first
glass-reinforced thermoset polymer substrate, portions of the
second glass-reinforced thermoset polymer substrate filling the at
least some of the plated-through-holes.
15. The printed circuit board as described in claim 14, wherein the
copper foil comprises circuit conductors.
16. The printed circuit board as described in claim 14, wherein the
shape of the copper flanges is selected from the group consisting
of diamond, triangular, square, rectangular, pentagonal, hexagonal,
octagonal, round, elliptical, and polygonal.
17. The printed circuit board as described in claim 14, further
comprising an additional copper plating situated on the filled
portion of the plated-through-hole, and on the copper foil.
Description
FIELD OF THE INVENTION
This invention relates generally to printed circuit boards. More
particularly, this invention relates to sequentially laminated
printed circuit boards having via structures.
BACKGROUND
Sequentially laminated printed circuit boards are multilayer boards
that contain blind vias. One or more sub-assemblies, for example
two two-layer boards or two four-layer boards, are first drilled,
plated, and patterned. Then the sub-assemblies are laminated
together and the entire assembly is drilled, plated, and patterned
again. The several plating steps required to plate the vias in this
multiplicity of steps produces relatively thick copper on the board
surfaces. This thick copper precludes the formation of fine circuit
traces when using isotropic wet etching. To avoid this problem, it
is common practice to reduce the copper thickness by mechanical
grinding or chemical etching. Neither process is highly uniform,
repeatable, or controllable. Referring now to FIG. 1, the copper
can easily be reduced too much, resulting in "butt joints" 140
between the barrel 120 of the via and the surface copper 130 at the
top edge of the via. Butt joints are latent defects that result in
intermittent contacts or open circuits when the laminate 110
expands faster during thermal excursions than the copper plating
120, causing the two metal members to separate. This results in
field failures, and it is highly desirable to eliminate this
reliability risk. It would be a significant addition to the art if
a multilayer printed circuit board could be fabricated that could
meet strict dimensional tolerances and still have highly reliable
via structures.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying figures, where like reference numerals refer to
identical or functionally similar elements throughout the separate
views and which together with the detailed description below are
incorporated in and form part of the specification, serve to
further illustrate various embodiments and to explain various
principles and advantages all in accordance with the present
invention.
FIG. 1 is a cross-sectional view of a via of a printed circuit
board as practiced in the prior art.
FIG. 2 is an exploded cross-sectional view of a printed circuit
board at an early stage in the process of forming a via structure,
in accordance with certain embodiments of the present
invention.
FIG. 3 is a cross-sectional view of a printed circuit board after a
first lamination, in accordance with certain embodiments of the
present invention.
FIG. 4 is a cross-sectional view of a printed circuit board after
drilling vias, in accordance with certain embodiments of the
present invention.
FIG. 5 is a cross-sectional view of a printed circuit board after
plating the vias, in accordance with certain embodiments of the
present invention.
FIG. 6 is a cross-sectional view of a printed circuit board after a
second lamination, in accordance with certain embodiments of the
present invention.
DETAILED DESCRIPTION
As required, detailed embodiments of the present invention are
disclosed herein; however, it is to be understood that the
disclosed embodiments are merely exemplary of the invention, which
can be embodied in various forms. Therefore, specific structural
and functional details disclosed herein are not to be interpreted
as limiting, but merely as a basis for the claims and as a
representative basis for teaching one skilled in the art to
variously employ the present invention in virtually any
appropriately detailed structure. Further, the terms and phrases
used herein are not intended to be limiting; but rather, to provide
an understandable description of the invention. The terms a or an,
as used herein, are defined as one or more than one. The term
plurality, as used herein, is defined as two or more than two. The
term another, as used herein, is defined as at least a second or
more. The terms including and/or having, as used herein, are
defined as comprising (i.e., open language). The term coupled, as
used herein, is defined as connected, although not necessarily
directly, and not necessarily mechanically. The terms "substantial"
and "substantially" are intended to have their ordinary meanings of
"largely but not wholly that which is specified," and are terms of
degree that should not be interpreted as having a strict numerical
limitation, implying "approximate" rather than "perfect".
A thick flange that will be incorporated in a via structure in a
later step is pattern plated on a sheet of copper attached to a
peelable support layer such as a thicker copper sheet with a thin
interfacial release layer. The copper surface is roughened to
enhance adhesion to a prepreg. This multilayer copper structure is
then laminated to a first major surface of the prepreg, while a
second such multilayer copper structure, a second ordinary copper
foil, or a printed circuit board subassembly comprising one or more
layers of copper and one or more layers of dielectric is laminated
to a second major surface of the prepreg. The support or carrier
layer is peeled away, leaving a copper sheet with thicker regions
embedded in the prepreg. The board is drilled and plated in the
conventional manner. The surface copper is then thinned by
mechanical or chemical means, without the risk of exposing the
copper barrel in the vias. Even if the entire thickness of field
copper is removed, the thick flange provides a highly reliable
joint. Optionally, a thin etch stop layer of a metal other than
copper, such as nickel, can be employed to ensure that the flange
is not etched away. Referring now to FIG. 2, a printed circuit
board (PCB) having via structures is created by beginning with a
prepreg laminate 210. Prepreg laminates are well known to those
skilled in the art, and are typically made from materials such as
glass reinforced epoxy, or glass reinforced polyimide. Prepregs are
thermoset materials, and are generally thin, ranging between 0.002
and 0.020 inches, and are generally processed while they are in a
deformable state, that is in a partially cured "B-stage". In this
partially cured state, the laminates are deformable, and will yield
and flow if significant pressure and/or heat is applied to them.
Once they are fully cured, they are generally considered to be
rigid and non-deformable.
Via lands or flanges 222 are pattern plated as pads on a sheet of
copper foil 224 that is attached to a peelable support or carrier
layer 228 (such as a thicker copper sheet) that can be subsequently
removed from the copper foil. The use of a thin release layer 226
at the interface aids in removing the carrier layer 228 from the
copper foil. These copper flanges will subsequently serve as a
collar around plated through holes that will be formed in the
laminate. Although the via lands or flanges are commonly circular,
they can also be polygons having 3 to N sides, where N is an
integer, or they can be irregularly shaped. For example, the copper
flanges can be shaped as diamond, triangular, square, rectangular,
pentagonal, hexagonal, octagonal, round, elliptical, or polygonal.
The lands therefore should be arranged in such a manner that they
correspond precisely to the location of the plated through holes
that will be subsequently drilled. The via lands or flanges 222 can
be formed in a number of ways, for example, patterned plated using
conventional photolithographic techniques. The flanges are plated
to a thickness that is greater than the thickness of the plating on
the walls of the plated through holes, typically 1 to 2 mils. In
one illustrative implementation, the thickness of the flanges is
equal to or greater than 1.5 times the thickness of the plating on
the walls of the plated through holes. In another illustrative
implementation, the thickness of the flanges is equal to or greater
than two times the thickness of the plating on the walls of the
plated through holes. The exposed surface of the plated copper
lands is then roughened using techniques such as mechanical
abrasion, chemical etching, and/or depositing additional materials
such as copper oxides, in order to increase the adhesion of the
flanges to the prepreg laminate in a subsequent lamination step.
The plated copper flanges on the carrier assembly 220 are then
bonded to one surface of the prepreg laminate, as indicated by the
arrows 215. Referring now to FIG. 3, during this step the copper
flanges 222 become embedded into the surface of the prepreg 210 by
the pressure and heat of the laminating press. Since the flanges
protrude above the surface of the copper foil, the prepreg is
deformed locally 325 around the flanges as it softens in the press.
The support or carrier layer 228 is then peeled away, leaving a
multilayer laminated structure that consists of the copper foil 224
bonded to the prepreg, the flanges 222 embedded in and bonded to
the prepreg, and the prepreg 210. Referring now to FIG. 4, holes
460 or apertures are then formed in this multilayer laminated
structure in conventional manner, such as mechanical drilling,
piercing, punching, or laser drilling. The holes extend completely
through the laminated structure, including the via lands or flanges
and the copper foil.
Since the alignment of plated through holes or via in a printed
circuit board is critically important with respect to the location
of other features such as circuit traces, the edges of the printed
circuit board, and other holes, fiducial holes 230 in both the
prepreg 210 and the copper foil 224 are used to align the foil to
the laminate. This ensures that the holes 460 drilled in the
laminate will be substantially concentric to the each of the
flanges 222. "Substantially" is intended to mean "largely but not
wholly that which is specified," and should not be interpreted as
having a strict numerical limitation, but instead implying
"approximate" rather than "perfect". Since the flanges are intended
to serve as a "reinforcement" around a top portion of the plated
thru hole, it is important that the holes in the prepreg be
accurately located with respect to the center line of the flange.
While it is not a requirement that the flange be perfectly
concentric to a central axis of the hole, dimensional tolerances
should be maintained to such a level that the flange ends up
located with respect to the hole such that most or all of the hole
falls within the outer circumference of the flange. The multilayer
structure is then plated (FIG. 5) in conventional manner in order
to form plating 562 along the walls of the drilled holes. During
the plating step, copper is also plated on the exposed surface 564
of the copper foil. Since a central portion of the flange has been
drilled out, copper is also plated on the interior walls of the
flange during the plating step. The plated copper on the walls of
the drilled holes forms a "barrel" 562 which extends completely
from one side of the multilayer structure to the other. During the
plating step, a portion 566 of the outer wall of the plated copper
barrel becomes attached to an interior vertical wall of the
flange.
In the fabrication of a multilayer printed circuit laminate 550, it
is important to maintain precise control of the thickness of each
layer, so that the resulting laminated structure will not exceed
certain dimensional tolerances. Also, it is desirable to have the
surface copper as thin as possible to facilitate the patterning of
fine copper lines and spaces by isotropic wet etching. For these
reasons, the plated copper 564 on the surface is often thinned or
reduced by a chemical milling or mechanical milling step. In prior
art systems this resulted in a thinned portion of copper around the
outer portion or "land" area, creating either a butt joint or a
very weak knee, as seen in prior art FIG. 1. In contrast, with our
invention, even in the most extreme case of milling, the embedded
flange 222 remains intact and the joint between the plated through
hole and the flange is reliable.
Referring now to FIG. 6, another layer of dielectric medium 610 can
be optionally applied to the laminated structure 550. Although
shown in the drawings on only the bottom side of the PCB, this
additional layer 610 can be placed on either or both sides of the
structure 550. In one embodiment, an additional layer of prepreg is
laminated to the multilayer structure on a side opposite the side
that contains the flanges. The additional layer of prepreg is
bonded to the original one using heat and pressure in conventional
means. During this laminating step, resin portions 660 of the
additional layer of prepreg flow into and fill the interior of the
plated through holes 560. Optionally, an additional layer of
plating 670, known as cap plating, is plated on the exposed portion
of the laminate over the filled portion of the plated through hole,
and optionally, above the flange. In one embodiment, the cap
plating 670 does not extend over exposed portions 625 of the
PCB.
In summary, without intending to limit the scope of the invention,
fabrication of a sequentially laminated printed circuit board
according to a method consistent with certain embodiments of the
invention can be carried out by pattern plating flanges or via
lands on a copper foil, laminating the foil to a prepreg so that
the flanges are embedded into the surface of the prepreg, creating
via holes in the laminate that are substantially concentric with
the individual flanges, plating the via holes with copper,
chemically or mechanically milling off a portion of the copper
plating and optionally some of the copper foil to reduce the
overall thickness of the laminate, and laminating a second and
optionally a third prepreg to the laminate. Those skilled in the
art will recognize that the present invention has been described in
terms of exemplary embodiments based upon use of glass reinforced
prepregs and copper plating. However, the invention should not be
so limited, since other variations will occur to those skilled in
the art upon consideration of the teachings herein. For example,
the copper foil that the flanges are formed upon can also be
patterned after lamination to the prepreg in order to form circuit
conductors and pads for attaching surface mount components such as
resistors, capacitors, integrated circuit packages, etc.
Accordingly, it is intended that the present invention embrace all
such alternatives, modifications and variations as fall within the
scope of the appended claims.
* * * * *